1. Field of the Invention
This invention is an aromatic diamine synthesis process. Aromatic diamines are used as intermediates in the synthesis of polyimide oligomers and polymers. The process is particularly useful for the synthesis of 2,2-bis(4-aminophenyl)hexafluoropropane which contains a hexafluoroisopropylidine (6F) group that is known to impart good processability, good thermal stability and good electrical insulation characteristics to the end product polyimide resins. Furthermore, the isolation of UV-vis chromopores by the 6F group allows the end product polyimides to appear colorless.
2. Description of the Prior Art
The chemical conversion of phenols to anilines is an inherently difficult task (see R. A. Scherrer et al. Journal of Organic Chemistry, Vol. 37, pg. 1681 (1972)) that is rendered more difficult when bis-phenols having inductively electron-withdrawing substituents are used as starting materials.
In particular, the chemical conversion of 2,2-bis(4-hydroxyphenyl)hexafluoropropane to other compounds is very difficult because the hydroxy functional groups on a molecule having the 6F groups are resistant to direct chemical modification. However, 2,2-bis(aminophenyl)hexafluoropropane has been prepared, in low yield, from 2,2-bis(4-hydroxyphenyl)hexafluoropropane that is commercially available under the trade name "Bisphenol AF" from DuPont Chemical Company of Willmington, Del. by K. Paciorek of Ultra Systems Inc. The preparation of 2,2-bis(4-aminophenyl)hexafluoropropane by Dr. Paciorek is described in NASA Report CR-159403 prepared by Ultra Systems Inc. of Irvine, Calif. (Ultra Systems Inc. Report No. SN 8,320-F). The process disclosed in NASA Report CR-159403 is unacceptable for practical use in that it involves strongly corrosive reagents and complex procedures which produce yields of less than 7%. Thus, there is still a need for a simple and efficient process for converting commercially available Bisphenol-AF, and other bis-phenol compounds, to dianiline compounds and there is a need for a simple and efficient process for the conversion of Bisphenol-AF to a dianiline compound having a 6F group between the two phenyl rings.